Physical Sciences Division Research Highlights

August 2009

Collision Science

How fast was that ion going when it hit?

Researchers at Pacific Northwest National Laboratory developed a new method that clocks the speed of ions as before and after hitting a thin wall of atoms. By calculating the speed, they can tell how much energy the ion loses in the collision, key information needed for technologies to advance energy, space exploration, and national security. Enlarge Image.

Results: When an ion hits a solid, the ion either knocks the atoms about or the ion transfers energy to electrons. While the first reaction is becoming fairly well understood, the second is less so. But Dr. William Weber and Dr. Yanwen Zhang at Pacific Northwest National Laboratory are working to change that. They recently developed an experimental approach that measures the energy lost to the electrons in the collision, a vital step in understanding what happens to the ion and the material it hits.

Why it matters: While ion collisions may seem the stuff of ivory towers, the new approach has implications for energy, space exploration, and national security. Inside nuclear reactors and nuclear waste, energetic ions are the primary mode of energy dissipation from nuclear reactions and radioactive decay. Understanding how the materials react is key to maintaining today's reactors and designing tomorrow's.

Cosmic radiation, another form of energetic ions in space, is an important consideration in designing materials and advanced electronics used in satellites and other space exploration technologies. In both these cases, the energy transfer to the electrons is not adequately known. Weber and Zhang's research is improving the understanding and modeling of the energy transfer processes in critical materials like silicon carbide.

As to national security, additional experiments using the new method provide insights into how materials give off light, or scintillate, when exposed to radiation. Collaborative work with researchers at Stanford University and a private company are helping identify which materials might be the best for gamma radiation detectors. These scintillation detectors will help find uranium and radionuclides that terrorists might try to bring into the country. The light emission from such radiation detector materials could provide a signature for the type of radioactive material present.

Further, a significant challenge is that testing new detector materials requires a time-consuming and expensive effort to develop growth techniques for large, flawless crystals. With Zhang and Weber's technique, researchers skip that step. Instead, they test with films that are just 15-micrometers thick.

Methods: The researchers found conventional scientific instruments and approaches could not measure the energy lost in the collision. The problem was the ions size and speed. Ions are so small; it would take up to tens of millions of them to cover the length of a dash - now that's small. As to speed, they can go as fast as several millions of miles a minute. So, Zhang and Weber designed their own approach.

To understand the approach, imagine throwing a football through a layer of ping pong balls. The football would start out very quickly, and slow down slightly after passing through the smaller balls.

Much like the football above, a single ion is hurtled at a thin layer of atoms. The speed of the single ion is measured as it moves toward the film and after it passes through. Measuring the difference between the starting and ending speed tells researchers how much energy was lost to the collision.

The researchers used this experimental technique to study the energy transfer between various ions and silicon carbide and zirconium dioxide films. Each measurement recorded 1 million ion events. This work was done at the Department of Energy's EMSL, a national scientific user facility at PNNL.

What's next: Zhang and Weber are working with their colleagues to understand the whole picture of ion-solid interactions, including where the energy transferred to the material goes.

Acknowledgments: DOE's Office of Basic Energy Sciences funded this work by Dr. William Weber and Dr. Yanwen Zhang of PNNL. The work was done in the Department of Energy's EMSL.